Particle separator

ABSTRACT

Apparatus for separating particles from a stream of gas and entrained particles comprising an annular duct having at one end an inlet to direct the stream into the duct such that the stream passes through the duct with swirl and at the other end outlet means to divide the inner part of the flow from particles in the outer region of the flow. The inlet and duct are shaped so that the swirl within the duct is that of a potential vortex in which the velocity is perpendicular to the axis of the vortex and is inversely proportional to the radius from this axis. This enables substantially streamline flow to be maintained. The annular duct may be conically tapered.

United States Patent Llewelyn et al.

PARTICLE SEPARATOR lnventors: Richard Penderell Llewelyn,

Cheltenham. Victoria; John Austin Hart, Burwood. Victoria. both of Australia State Electricity Commission of Victoria Melbourne. Victoria. Australia Filed: Dec. 11, 1972 Appl. N04: 3l4,l65

Assignee:

Foreign Application Priority Data Dec. 9. 1971 Australia 7328/7l References Cited UNITED STATES PATENTS 9/l904 Hollingsworth 55/459 5/1907 Miller 55/459 1 May 13,1975

2.378 6U(l (i/I945 Tongeven .c 55/399 2.385.745 9/l945 VOgl i a c .4 55/392 2,560,069 7/l95l Bloomer 55/459 2.665.809 l/l954 Chisholm i a 209/2l l 1060.664 l0/l962 Morawski i t 55/338 3,488,924 l/l970 Reeve 209/21 l 3.5l3 642 5/[970 Cornett i i i t i A 55/5l8 3 7 l6 967 2/l973 ODoyle Jr.. et alv 7. 55/459 Primary Examiner-Bernard Nozick [57} ABSTRACT Apparatus for separating particles from a stream of gas and entrained particles comprising an annular duct having at one end an inlet to direct the stream into the duct such that the stream passes through the duct with swirl and at the other end outlet means to divide the inner part of the flow from particles in the outer region of the flow. The inlet and duct are shaped so that the swirl within the duct is that of a potential vortex in which the velocity is perpendicular to the axis of the vortex and is inversely proportional to the radius from this axis. This enables substantially streamline flow to be maintained. The annular duct may be conically tapered.

3 Claims, 9 Drawing Figures FIJENIEQ HAY I 3 i875 SHEET 10F 4 PARTICLE SEPARATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the separation of particles from a stream of gas and entrained particles. As used herein, the term particles concentration" signifies a measure of the quantity of particulate material to the quantity of gas in the stream or a part thereof on a weight or volume basis and not of the actual number of particles in the stream. Thus, a stream which has a high particle concentration is rich in particulate material and lean in gas whereas a stream of low particle concentration is relatively rich in gas and lean in particulate material.

2. Description of Prior Art One application of the invention is to the burner pipes of brown coal burning boilers. In such boilers, the brown coal is pulverised in a mill and is carried by a stream of hot gas to the burners, the mill providing the active mixing needed for drying. The stream of gas and pulverised fuel is usually passed through a classifier before being supplied to the burner so that oversize fuel is returned to the mill for further treatment. With the high moisture content of the brown coal, large quantities of water vapour are generated which tend to quench the flame. This problem can be overcome by causing a partial segregation of the pulverised fuel from the accompanying gas and water vapour to form separate sub-streams of differing particle concentration, these sub-streams being supplied to separate parts of the furnace to improve the combustion and heat transfer therein.

Conventionally the required partial segregation for separation firing is obtained in a tubular concentrator fitted with radial vanes which superimpose on the flow a swirliing motion. Such a concentrator creates large pressure losses in the system and substantial extra power is required to overcome these losses. Moreover the turbulence associated with the swirl tends to oppose the inertial separation of the coal particles.

By the present invention it is possible to create three dimensional swirling flow which is also a substantially streamline or irrotational flow. Such a flow can provide efficient separation of particles without turbulent remixing and with much lower power losses than with conventional cyclone separators.

SUMMARY OF THE INVENTION The irrotational flow on which the present invention is predicated is the potential vortex in which the velocity is perpendicular to the straight axis of the vortex and is, inversley proportional to the radius from this axis, the constant of proportionality being the vortex strength. There must also be transmission of the flow parallel to the vortex axis. The degree to which the real flow will correspond to the theoretical potential vortex will depend on the extent to which the flow is influenced by boundaries. The rate ofgrowth of wall boundary layers can be reduced by accelerating the flow along these boundaries If a sink is located on the vortex axis to generate the required fluid translation parallel to it the stream lines of the flow will lie along the surfaces of the cones having apices at the sink location and it is therefore possi ble and preferred to select the frustums of two such cones as the inner and outer physical boundaries. The

flow speed at these boundaries is everywhere increasing in the direction of motion of the flow producing favourable conditions for the wall boundary layers. The separating effect in the vortex flow will tend to throw the particles towards the outer cone; at the inner cone the small radius and high swirl component of the fluid velocity together tend to produce a large centripetal acceleration and hence separating effect just where it is most advantageous.

According to the invention therefore there is provided a method of separating particles from a stream of gas and entrained particles, comprising causing the stream to flow with substantially a potential vortex swirl about a straight vortex axis and with movement along the axis so that the particles tend to outer regions of the flow and dividing an inner part of the flow from those particles in the outer regions of the flow.

The invention further provides apparatus for separating particles from a stream of gas and entrained particles, comprising an annular duct having at one end an inlet to direct the stream into the duct such that the stream will pass through the duct with substantially potential vortex swirl about the central axis of the duct and at the other end outlet means to divide an inner part of the flow from particles in the outer region of the flow.

In order that the invention may be more fully explained, some specific separators and mathematical methods by which components of those separators can be designed will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings,

FIG. 1 is a perspective view of a conical vortex sepa rator fitted with a contracting inlet of one specific design;

FIG. 2 is a perspective view of a conical vortex separator fitted with a contacting inlet ofa different design;

FIG. 3 is a partly sectional view of the vortex separator shown in FIG. 2;

FIG. 4 is a diagrammatic longitudinal cross-section through a conical vortex separator incorporated in a burner for a furnace;

FIG. 5 is a diagrammatic longitudinal cross-section through a conical vortex separator used as a particle collector for gas cleaning purposes;

FIG. 6 shows the axes of Cartesian, cylindrical polar and spherical polar co-ordinate systems used in the mathematical design methods to be described;

FIG. 7 is a conceptual sketch of the flow in a conical vortex separator also to be referred to in the descrip tion of the mathematical design methods;

FIG. 8 is a further sketch showing co-ordinates and parameters considered in the mathematical analysis; and

FIG. 9 is a sketch showing (to-ordinates and parameters considered in the design of the inlet.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 shows a conical vortex separator comprised of a contracting inlet 11 and a conically tapered annular duct 12. The inner wall 13 and the outer wall 14 of annular duct 12 are both conically tapered to converge toward a common origin disposed on the central axis of the duct and beyond the outlet end 16 of the duct.

in use of the separator a stream of gas and entrained particles is passed into inlet 11 whence it is directed from a uniform streamline flow into a swirling flow having swirl about the axis of the annular duct 12. The swirling flow passes along the duct to the end 16 and gas is drawn from an annulus comprising the inner part of the exit annulus of the flow at the duct end 16. in this manner there is generated a flow which conforms to that defined by a point three-dimensional potential sink lying on the axis of a potential vortex. the sink being located at the apices of the cones which constitute the stream surfaces of such a flow. More particularly the entry of the stream from the inlet 11 into the duct 12 is directed relative to the sink such that there is established within the duct 12 a substantially potential sinkvortex flow having bounding streamlines which lie on the surfaces of cones converging to the same origin as the conical walls 13. 14. Thus the walls 13, 14 conform to the conical stream surfaces of the potential sinkvortex flow and can serve as inner and outer physical boundaries with very little generation of turbulence. The flow speed on these boundaries will increase in the direction of motion of the gas producing favourable conditions for the wall boundary layers.

In the vortex flow within duct 12 the highest veloci ties occur where the particle concentration is least and vice versa. This is clearly favourable from the point of view of wear on the walls l3, l4. Turbulence will of course be present to some degree but it is known that turbulent mixing is enfeebled at convex surfaces and strengthened at concave surfaces These results are favourable for the swirl separator since the tendency to oppose separation will be minimised in the inner flow and is of little consequence in the outer flow where in any case the high concentration ofparticles will tend to damp turbulent motion.

FIGS. 2 and 3 illustrate a conical vortex separator similar to that shown in FIG. I but with a contracting inlet of slightly different shape. In this case the annular duct 17 has cylindrically curved outer and inner wall portions l8, 19 extending from the inlet 21 to the main conically tapered outer and inner wall portions 22, 23 of the duct. The inlets ll and 21 in both cases are of contracting cross-section in the direction of flow and are shaped to produce at the inlet to the annular duct a radial distrubution of circumferential velocity conforming to the potential vortex within the duct. a radial distribution of axial and radial velocity conforming to the flow component producing movement along the axis and having a cross-sectional shape such that the flow adjacent the side of the iniet most remote from the outlet end of the annular duct will, after one revolution in this duct, exactly coincide with the flow emerging from the inlet adjacent the side nearest to the outlet end of the annular duct and have a matching velocity distribution so that the flows join smoothly. lo the separator illustrated in FIG. 1 the side of the inlet most re mote from the outlet end 16 of the annular duct 12 is indicated as 24 and the side nearest to the outlet end 16 is indicated as 26. For convenience these will be termed respectively the roof" and the floor" of the inlet although it is to be understood that such terminology does not imply that the separator can only be operated in the upright position shown in FIG. 1. As will be apparent from the embodiment illustrated in H0. 4 the separators according to the invention can be operated in other dispositions. The transition from uniform parallel inlet flow to the sink-vortex flow occurs along the inlet duct upstream of a plane at a position indicated by the dotted line 27 which. if extended, contains the central axis of the separator. Downstream of this plane the flow conforms to the swirling sink-vortex flow in the annular duct.

The inlet end of the annular duct is finished to a spiral conforming to the vortex flow and is fitted with an annular roof 31 which joins smoothly with the roof 24 of the inlet. Roof 24 and floor 26 of the inlet are connected by an outer wall 25 and an inner wall 30. Outer wall 25 smoothly joins the outer tubular wall 14 of the annular duct at the entrance to the duct defined by the dotted line 27. Inner and outer walls 25, 30 converge in the direction of flow through the inlet and the inlet as a whole is of contracting cross-section in the direction of flow.

The roof 24 and floor 26 of the inlet are non-parallel. More particularly they are angled so that the flow which crosses the duct entrance adjacent the roof 24 will, after one revolution within the annular duct, exactly coincide with the flow adjacent floor 26 at the duct entrance 27. Thus the duct roof 31, after one revolution around the duct, joins smoothly with the floor 26 of the inlet at the duct entrance. Because of the contracting cross-section of the inlet and the angling of its floor and roof the flow leaving the inlet at the duct entrance 27 has a swirl component of velocity which is inversely proportional to the distance from the central axis of the duct with superimposed velocity along the duct and the flow adjacent the inlet floor 26 will have the same velocity as the flow in the duct adjacent duct roof 31 so that the flows in the inlet and the duct join smoothly with minimum turbulence, Thus after passing into the annular duct the flow can continue with its sinlovortex swirling flow without the need for any physical dividers within the annular duct.

The separator of FIGS. 2 and 3 is similar in operation to that of FIG. 1 in that there is a radial plane marking the transition between the inlet flow and the flow within the annular duct. in this case the transition is at a position where the floor 28 of the inlet joins the duct roof 32. As in the previous case duct roof 32 is a smooth continuation of the inlet roof 29. The roof 29 and floor 28 of the inlet are connected by an outer wall 3 and inner wall 34. As before. the roof and floor of the inlet are non parallel and their mutual inclination varics in the direction of flow to enable the smooth join ing ofthe continuation of the roof with the floor of the inlet at the entrance to the duct. Also as before, the inlet is of contracting cross-section in the direction of flow with the result that at the entrance to the duct the flow leaving the inlet has a swirl component of velocity which is inversely proportional to the distance from the central axis of the duct with a superimposed velocity along the duct and the flow adjacent the inlet floor will have a same velocity as in the flow in the duct against the duct roof so that the flows of the inlet and the duct join smoothly with a minimum turbulence. The re quired spiral swirling flow is thus established within the duct without the need for any physical dividers within The niathematicai methods whereby the annular ducts l2. l7 and the contracting inlets ll, 21 can be designed will be explained hereafter.

Separators of the types shown in FIGS 1 to 3 could simply replace the more conventional separators presently used in particle separation firing of boiler installations. Alternatively. in order to use the angular motion of the fuel rich stream. the separator could be attached directly to the furnace wall and the fuel stream discharged as from a swirl burnerv This could be done while still obtaining the full benefit of complete separation by returning the fuel-lean vapour stream inside the inner cone of the separator.

An exemplary combination separator and burner is shown in FIG. 4. The combination burner and separator 40 is mounted in the furnace wall 41. The inner and outer conically tapered walls of the separator are designated as 42, 43 and the fuel and vapour stream enters the end 44 of the duct between these walls from a contracting inlet (not shown).

The burner end of the combination has a refractory nose 45 which is shaped to provide a return passage 46 through which fuel-lean vapour is drawn from the inner region of the swirling flow in the separator duct and to waste through a central tube 47. Straightening vanes 48 are provided at the entrance to the return passage 46 to give pressure recovery of the swirl energy present in the waste vapour. Since the waste gases come from the most energetic part of the flow within the separator (i.e., adjacent the inner wall) such pressure recovery is quite important.

The particle enriched portion of the stream at the outer wall of the separator passes from the separator with swirling motion and is mixed with secondary air from a secondary air manifold 49 as it discharges di' rectly into the furnace for combustion.

Separators according to the invention may also be used for gas cleaning purposes and FIG. 5 illustrates a separator 50 for separating and collecting particles from a gas stream. The inner and outer conically tapered walls of the separator are designated as 51 and 52 and the particle laden gas stream enters the inlet end 53 of the duct from a contracting inlet 54. In this case a conical extension 56 of the outer wall 52 of the annular duct is provided so as to define a particle collection chamber 57 beyond the annular duct outlet.

Particle lean gas is drawn from the inner part of the swirling flow at the duct outlet via an annular inlet 58 to a gas outlet pipe 59 extending back through the interior of the annular duct. The wall portion 61 defining the central aperture through annular inlet 58 is connected to a further pipe 62 extending back through the interior of the pipe 59 so that gas may be drawn from chamber 57 through this pipe 62. Adjacent the inlet end of the annular duct pipe 62 passes out through the wall of pipe 59 and connects with inlet 54 at 63. Gas flowing into the annular duct via inlet 54 causes a pressure reduction in pipe 62 so that gas is drawn from chamber 57 back through pipe 62 and into inlet 54 so as to be recycled through the annular duct. Inlet 58 is fitted with flow straightening vanes 64 to give pressure recovery of the swirl energy present in the gas entering outlet pipe 59.

Particles in the outer regions of the swirling flow within the annular duct pass from the duct outlet into chamber 56. They settle in the bottom end of the chamher which serves as a collection hopper and are removed by intermittent or slow continuous rotation of an eliminator valve 66.

Separators of the type illustrated in FIG. 5 may find wide use in gas collecting applications. They could be used in place of electro-static separators or as precleaners in advance of electro-static cleaners.

DESCRIPTION OF MATHEMATICAL DESIGN METHODS I. Introduction The general problem of three-dimensional particle motion is. of course, highly complex; however, it is possible to derive approximate analytical expressions for the trajectories of very small particles in simple swirling flows. Provided that the smallest particles of interest are indeed very small in this context. and that the simple flow postulated can be approximated in practice, the solutions obtained in this way can be applied to the design of devices in which these, and all larger particles, are separated from a portion of the gas.

2. Nomenclature Symbols used in one section only are defined as they occur and are not listed here.

A bar over a symbol denotes a vector quantity.

A bar over a mean axial velocity at separator exit 2 co-ordinates of separator entry and exit planes axes of Cartesian coordinate system )See )Figure axes of cylindrical polar coordinate system )6 axes of spherical polar coordinate system) See Figure 6 semi-angles of inner and outer separator cones fluid density particle density fluid viscosity kinematic fluid viscosity u/p vector differential operator 3. Equation of particle motion The inertia of dense particles carried by a gas flow causes them to diverge from curved fluid streamlines The centripetal force arising from the pressure gradient in an irrotational flow is sufficient to produce the required lateral acceleration of an element of gas, but not of a denser solid particle, which therefore pursues a straighter path and thus acquires a velocity relative to the gas. The particle is then subjected to gavitational, pressure gradient and drag forces. The resultant of these forces can, by Newtons second law, be equated to the product of the mass and acceleration of the particle, and the equation of motion integrated within a prescribed flow field to determine the particle trajectory.

The equation of motion for a single spherical particle whose position vector is 'r' is Putting ft C particle drag coefficient T- .1 mm Re particle Reynolds numher F t l l drpar )6 e e may ml C equation l may be written To solve equation (3) it is convenient to select, from the cylindrical (z, r, 6) and spherical (R, dz, 6) polar coordinate systems shown in FIG. 6, the components in the z. :1), and directions. The relevant equations are then where a and b are unit vectors in the R and 9 directions.

5. Solution of the equation of motion in sink-vortex flow The method of solution depends on the fact that for very small particles the parameter C is much less than unity.

It is assumed that terms of O(C or higher can be neglected.

For equations (6) and (7) a linear equation in r dB/dt whose solution is From equations (4), (5), (7) and (9) It is to be expected that small particles will diverge slowly from the fluid streamlines, so that it is likely that the second derivatives will be small compared with first derivatives. We therefore take as first approximations from which, by differentiation,

and hence by substituting back into equations and separated, it can be used to derive the fraction of the (11) obtain the second approximations flow free of these and all larger particles, as the latter will have experienced a greater lateral movement.

FIG. 7 is a conceptual sketch of the sink-vortex sepad; M cosdi ZCM2 cos"d q +O(C) 5 rator.

The total volume flow is dd) CK? cosdz 2 dr r. sin dw z tame; Finally, to O(C) I0 0 Q w 21rr dr 11 CK2 cos di T- M 2 a Z talltm The same result obtains if the term in C is neglected 15 from the approximation to dz/dr, from which it appears that the axial particle velocity is, for all practical purpose, equal to the axial component of the fluid velocity.

where, since the velocity potential Thus the particle only possesses a velocity relative to d: the fluid in the r direction. "l

Takin g the vertical velocity is 2 M T i M,

l k W: 52 (--+r' d4: CKcosdr dr z sin hence Q 21rM (cosdn, COS H7) 11 M simb CK2 T (ztan)=T+ 7'' Consider a particle which traverses the flow from 4) hence [13) to d) between 1,, and Z Then the volume flow 211M (cosqS cosd contains only particles smaller than this particle in the plane z 1,. Therefore, if the separation Thus, the separating effect is entirely due to the vortex b w en and z, ith respect to a particular size of particles is defined as the fraction of volume flow comlntegratlon of equation (12) gives pletely clear of those particles.

2 S cosda cosd: 8 CK i Seed: cos-(b M2 q Cow cosrb Choosing now the particle path which originates at From equations (16) (17) and (18) the inner cone qb in the plane 1 z, 605% x S cosda cosd: l9)

z 1 1 1 CK l l secd costb M T secrfi cosaS 14] Where X M 21 2n u (103% or The expression VX l in equation l9) simplifies (K2 I l cosd: T SBCQD cos cosda l 0 l5 The appropriate solution is to 1 ZCK2 l l V tsec cosdz l (sec 45., cos tb 1" 6. Design equation for a sink-vortex separator when the term in C is neglected. lf Equation (14) will be valid for very small particles only, but if it is applied for some specified value of C, 2CK 1 1 corresponding to the smallest particles required to be (secificos M T (64hr cow") l O f) w n THEORY OF INLET DESIGN The conical section of a conical vortex separator is shown in FIG. 8. Streamlines of the sink-vortex flow are defined by Rtfd) RsinqSdO (K/Rsintbl from which b constant, that is, streamlines lie on the surfaces of cones with apices at the origin 0', and

The top surface of the separator is defined by tracing back through a full revolution 6 21r streamlines which intersect the radius 9 6... ABCD is then the cross-section required of an inlet to the sink-vortex flow. Ducting upstream is to be so shaped as to produce at this section the three-dimensional velocity distribution appropriate to the sink-vortex flow.

The velocity field of a two-dimensional vortex flow can be produced from a curved, contracting flow designed by the Helmholtz-Kirchhoff method of potential flow analysis. The bounding streamlines of the resultant flow are shown in FIG. 9. On the continuously-curved outer streamline the flow speed is constant and equal to V,; on the inner streamline the speed increases from V to along the straight section between 0) and (u, 0) and there is thereafter constant. By so specifying the speed on the inner boundary a smoothly curved shape is obtained with the angle of the surface increasing continuously with distance along it. In an alternative. somewhat more compact design, the straight section on the inner boundary is preceded by a curved section having the same speed as on the outer boundary; this results in a bump" projecting into the flow on the inner wall. The use of these designs is illustrated in FIGS. 2 and 3 and FIG. 1 respectively.

The two-dimensional flow in FIG. 9 is scaled to correspond with the section of the separator in the xy plane z z of FIG. 8, that is, r z tandr. V(r) Klz tanqb. The radius 6 0 is chosen such that the streamlines crossing it are, to a sufficient approximation. circular.

It remains to add to the velocity at each point in the two-dimensional flow, denoted by its x and y components u..(.r, y) and v .(x. y), the sink velocity M/R at that point. The x. y and z components of the velocity at the point (x. y 2) in the inlet duct are then w Nil/R where The streamlines are defined by where The streamline through any point in the section ABCD in FIG. 8 is traced back by a marching integration procedure sufficiently far to define a point in essentially parallel. uniform flow. The process is repeated for a number of starting points on the boundary ABCD to define the duct shape.

The use of a uniform velocity of translation parallel to the vortex axis rather than that appropriate to a sink simplifies the calculations, in particular leading to a cylindrical rather than conical separator. In this case the calculated (2:, y) co-ordinates of streamlines in the twodimensional contracting flow are preserved in corresponding streamlines in the three dimensional inlet duct, so that it is only necessary to calculate the progressive increase in z of streamlines traced back from their intersection with the radius 6 6 in the xy plane z 1 The z dimension of the duct between corresponding streamlines in its upper and lower surfaces re mains constant In a conical vortex separator in which the cone angles are small it may be sufficient to use a cylindrical inlet, designed with W equal to the average vertical velocity in the xy plane 2 20.

We claim:

1. Fuel delivery means for a furnace fired by gas borne particulate fuel. comprising an apparatus for separating particles from a stream of gas and entrained particles, said apparatus comprising:

an elongated annular duct having an outer tubular wall and an inner tubular wall both extending from an inlet end of the duct through to an outlet end of the duct;

an annular duct roof on the inlet end of the duct;

an inlet to the duct to direct said stream into the inlet end of the duct so that it flows with swirling motion through to an outlet end of the duct; and

outlet means to divide an inner part of the flow at the outlet end of the duct from particles in the outer region of the flow; wherein said outlet means comprises passage means having an annular inlet presented to the inner part of the flow at the exit annulus of the duct which passage means extends from its inlet duct through the space within the inner wall of the annular duct to the inlet end of the duct; and wherein said inlet has a roof, a floor and inner and outer walls connecting the roof and floor so as to define a single inlet passage separate from the annular duct, the roof and floor of the inlet are non-parallel. and the roof of the inlet joins smoothly onto the roof of the duct which latter roof joins smoothly with the floor of the inlet at the entrance to the annular duct; and

the inner and outer walls of the inlet converge in the direction of flow and smoothly join the inner and outer walls of the duct respectively. whereby at the entrance of the annular duct the flow in the inlet enters the duct smoothly;

such apparatus being mounted exteriorly of the furnace wall with said annular duct transverse to the wall and said outlet end of the duct disposed within an opening through the furnace wall such that said particles in the outer regions of the flow at the outlet end of the duct pass into the furnace via said opening with swirl from the duct.

2. Fuel delivery means as claimed in claim 1, further comprising an annular supply manifold disposed about the outlet end of the duct to supply air to the furnace via said opening so as to mix with the particles entering the furnace 3. Apparatus for separating particles from a stream of gas and entrained particles, comprising:

an elongated annular duct having an outer tubular wall and an inner tubular wall both extending from an inlet end of the duct through to an outlet end of the duct; and

the inner and outer walls of the duct both converge conically toward a common convergence apex beyond the outlet end of the duct so that the duct is of Contracting annular crosssection toward its outlet end;

an annular duct roof on the inlet end of the duct;

an inlet to the duct to direct the stream into the inlet end of the duct so that it flows with swirling motion through to an outlet end of the duct; and

outlet means to divide an inner part of the flow at the outlet end of the duct from particles in the outer region of the flow; wherein said inlet has a roof, at floor, and inner and outer walls connecting the roof and floor so as to define a single inlet passage separate from the annular duct. the roof and floor of the inlet are non-parallel, and the roof of the inlet joins smoothly onto the roofof the duct which latter roof, after one revolution around the duct joins smoothly with the floor of the inlet at the entrance to the annular duct; and

the inner and outer walls of the inlet converge in the direction of flow and smoothly join the inner and outer walls of the duct respectively, whereby at the entrance of the annular duct the flow in the inlet enters the duct smoothly. 

1. Fuel delivery means for a furnace fired by gas borne particulate fuel, comprising an apparatus for separating particles from a stream of gas and entrained particles, said apparatus comprising: an elongated annular duct having an outer tubular wall and an inner tubular wall both extending from an inlet end of the duct through to an outlet enD of the duct; an annular duct roof on the inlet end of the duct; an inlet to the duct to direct said stream into the inlet end of the duct so that it flows with swirling motion through to an outlet end of the duct; and outlet means to divide an inner part of the flow at the outlet end of the duct from particles in the outer region of the flow; wherein said outlet means comprises passage means having an annular inlet presented to the inner part of the flow at the exit annulus of the duct which passage means extends from its inlet duct through the space within the inner wall of the annular duct to the inlet end of the duct; and wherein said inlet has a roof, a floor and inner and outer walls connecting the roof and floor so as to define a single inlet passage separate from the annular duct, the roof and floor of the inlet are non-parallel, and the roof of the inlet joins smoothly onto the roof of the duct which latter roof joins smoothly with the floor of the inlet at the entrance to the annular duct; and the inner and outer walls of the inlet converge in the direction of flow and smoothly join the inner and outer walls of the duct respectively, whereby at the entrance of the annular duct the flow in the inlet enters the duct smoothly; such apparatus being mounted exteriorly of the furnace wall with said annular duct transverse to the wall and said outlet end of the duct disposed within an opening through the furnace wall such that said particles in the outer regions of the flow at the outlet end of the duct pass into the furnace via said opening with swirl from the duct.
 2. Fuel delivery means as claimed in claim 1, further comprising an annular supply manifold disposed about the outlet end of the duct to supply air to the furnace via said opening so as to mix with the particles entering the furnace.
 3. APPARATUS FOR SEPARATING PARTICLES FROM A STREAM OF GAS AND ENTRAINED PARTICLES, COMPRISING: AN ELONGATED ANNULAR DUCT HAVING AN OUTER TUBULAR WALL AND AN INNER TUBULAR WALL BOTH EXTENDING FROM AN INLET END OF THE DUCT THROUGH TO AN OUTLET END OF THE DUCT; AND THE INNER AND OUTER WALLS OF THE DUCT BOTH CONVERGE CONICALLY TOWARD A COMMON CONVERGENCE APEX BEYOND THE OUTLET END OF THE DUCT SO THAT THE DUCT IS OF CONTRACTING ANNULAR CROSS-SECTION TOWARD ITS OUTLET END; AN ANNULAR DUCT ROOF ON THE INLET END OF THE DUCT; AN INLET TO THE DUCT TO DIRECT THE STREAM INTO THE INLET END OF THE DUCT SO THAT IT FLOWS WITH SWIRLING MOTION THROUGH TO AN OUTLET END OF THE DUCT; AND OUTLET MEANS TO DIVIDE AN INNER PART OF THE FLOW AT THE OUTLET END OF THE DUCT FROM PARTICLES IN THE OUTER REGION OF THE FLOW; WHEREIN SAID INLET HAS A ROOF, A FLOOR, AND INNER AND OUTER WALLS CONNECTING THE ROOF AND FLOOR SO AS TO DEFINE A SINGLE INLET 